The grey goo nightmare is a hypothetical end of the world scenario where self-replicating molecular nanobots or nanomachines consume all the matter on Earth, turning it into more nanomachines. The term "grey goo" was coined by Eric Drexler in his 1986 book _Engines of Creation_.

The grey goo scenario is based on the idea that nanotechnology could advance to a point where molecular machines could become self-replicating, and that if they were to escape from a controlled environment, they could multiply uncontrollably, consuming all available resources and potentially causing a global catastrophe.

The grey goo scenario remains controversial and highly speculative, with many experts arguing that the risks are overstated. Nonetheless, it has captured the public imagination and has been featured in numerous works of science fiction and other popular media.